FIELD OF THE INVENTION
The invention relates to a polarity reversal protection circuit that includes a first semiconductor switch and a polarity reversal protection diode that are connected in series with a load and that includes a second semiconductor switch that is connected in parallel with the polarity reversal protection diode. When the supply voltage terminals are properly connected to the circuit, the second semiconductor switch is switched on at least during the times that the first semiconductor switch is turned on.
The problem with power semiconductor switches and, in particular, MOS semiconductor switches by comparison with a relay is its diode, dictated by the technology, between the source connection and the drain connection of the MOS semiconductor switch. This diode, which is integrated in the MOS semiconductor switch and is also referred to as an inverse diode, is forward biased in the event of polarity reversal of the operation voltage and thus leads to the current flow through the load elements connected to the source connection or drain connection. In order in this case to afford effective protection of the load elements and of the MOS semiconductor switch against polarity reversal, it is regularly the case that a polarity reversal protection circuit is connected in series with the MOS semiconductor switch. In this case, the polarity reversal protection diode is biased such that during normal operation, that is to say correct connection of the supply voltage to the terminals of the circuit configuration, a current flow can take place through the polarity reversal protection diode. In contrast, the polarity reversal protection diode is in the off state in the event of polarity reversal, that is to say when the operational voltage is incorrectly connected to the terminals of the circuit configuration.
FIG. 1 illustrates a circuit configuration which is suitable for this purpose. The operating voltage V can be applied to two terminals 2, 3 in order to supply a load 16 with voltage, in a manner controlled by a semiconductor switch 12. By way of example, the load 16 may be the lamp of a motor vehicle headlight, which is switched on as required by the driver. In detail, the series circuit formed by the polarity reversal protection diode 10 already mentioned, the semiconductor switch 12 and the load 16 is present between the terminals 2 and 3. The anode connection of the polarity reversal protection diode 10 is connected to the terminal 2 and the cathode connection of the polarity reversal protection diode 10 is connected to the drain connection D of the semiconductor switch 12, which is designed as a MOS semiconductor switch in the present case. The source connection S is connected to one terminal of the load 16, the other terminal of which is connected to the terminal 3. An inverse diode 14 is connected in parallel with the load path of the semiconductor switch 12 and thus between the drain connection D and source connection S thereof. The anode connection of the inverse diode 14 is connected to the source connection S and the cathode connection is connected to the drain connection D. The semiconductor switch 12 is controlled at its control connection or gate connection G by a control device 20, which is known sufficiently to a person skilled in the art.
The circuit configuration illustrated in FIG. 1 shows a so called highside switch. Specifically, the semiconductor switch 12 is connected to the positive pole of the supply voltage terminal during normal operation by way of the polarity reversal protection diode 10. FIG. 1 illustrates normal operation, that is to say that the positive pole of the supply voltage V is connected to the terminal 2 and the negative pole to the terminal 3.
What is problematic with the circuit configuration illustrated in FIG. 1 is the power loss that is caused by the polarity reversal protection diode 10 and that is consumed continuously during normal operation because the current flow must necessarily flow through the forward biased polarity reversal protection diode 10. The power losses are extremely high primarily when the semiconductor switch 12 is intended to switch high currents. In this case, the voltage drop across the polarity reversal protection diode 10 is often unacceptably high, because this reduces the effective voltage across the load 16.